This invention is related, generally, to the field of fiber optic lasers and, more specifically, to fiber optic lasers that operate in a shorter than normal wavelength range.
Fiber optic lasers are known in the art, and are used as coherent optical sources for a number of applications. An optical fiber, often doped with an active material such as a rare earth element, is xe2x80x9cpumpedxe2x80x9d by coupling optical energy into it at a predetermined wavelength. The pumping wavelength is selected according to the characteristics of the fiber, such as an absorption band of a particular dopant that has been used. Pumping energy coupled into the fiber is absorbed, causing a population inversion in the doped material that is followed by optical radiation at a characteristic wavelength of the doped fiber. By providing the fiber with reflective ends, a resonance condition develops within the fiber cavity. An output coupler, being only partially reflective, allows the output of the developed laser energy.
Many fiber optic lasers use a single mode fiber, and couple the pumping energy of the laser directly into the fiber core. Alternatively, the fiber may be a double-clad fiber in which the core is surrounded by two different cladding layers, an inner or xe2x80x9cpumpxe2x80x9d cladding surrounding the core, and an outer cladding surrounding the pump cladding. Fibers of this configuration may be pumped by injecting the pump energy into the pump cladding layer of the fiber. The pump energy repeatedly encounters the core of the fiber as it undergoes internal reflection within the pump cladding layer, and is absorbed by the dopant of the core, providing the desired population inversion. The use of a double-clad fiber typically allows the coupling of a greater amount of pump energy into the laser than is possible with a single-cladding fiber. As such, double-clad fiber lasers are desirable for higher power laser applications.
One example of a cladding-pumped fiber laser uses a double clad fiber doped with ytterbium (Yb). Like other rare earth doped optical fibers, the Yb doped fiber laser has a xe2x80x9cgain bandwidthxe2x80x9d of optical wavelengths that are generated by the doped fiber when it is pumped. Often it is desirable to operate the laser at a narrow band of output wavelengths, and the laser may therefore be stabilized to resonate over such a wavelength range. Typically, such stabilizing is accomplished using a wavelength selective element, such as a fiber grating, in the laser cavity. The result is a high power narrow bandwidth laser.
Certain applications exist for which it is desirable to provide laser energy at a wavelength toward the shorter end of a given fiber laser gain spectrum. For example, it is desirable in some instances to operate a Yb-doped cladding-pumped fiber laser at a wavelength between 1060 nm and 1090 nm. Typically, such a laser is stabilized to the desired output wavelength by a high-reflectivity input grating.
The present invention provides a fiber laser that, for a given temperature range, is more stable than those of the prior art. It has been discovered that, as the temperature of the fiber laser increases (due, for example, to a rise in ambient temperature, or additional heating resulting from an increase in pumping power), the laser becomes unstable. In the case of a Yb-doped laser stabilized at 1080 nm, the instability manifests itself as an additional wavelength peak at about 1095 nm. Of course, this significantly reduces the usefulness of the laser for its intended application.
In accordance with the present invention, a fiber laser is therefore provided with a gain spectrum control mechanism that maintains wavelength stability in the apparatus by preventing a thermally-induced shift of the gain spectrum to a wavelength range that is unstable for the desired fiber laser wavelength (also referred to herein as the xe2x80x9csignal wavelengthxe2x80x9d). That is, the laser is operated so that the gain spectrum for which the instability problem arises is not encountered. The two general ways of providing this control are by shifting a nominal gain spectrum of the laser away from the less stable wavelength range, and by preventing a shift of the gain spectrum by controlling the operating temperature of the laser. The laser may use any of a number of different types of optical fibers, but in the preferred embodiment it is a multiple clad fiber and, more particularly, a double clad fiber.
When the nominal gain spectrum of the laser is shifted sufficiently away from the less stable wavelength range, the gain spectrum can drift with temperature without encountering the less stable range. For example, the reflectivity of the resonant cavity may be reduced below the typical level. As such the laser must operate at a higher gain, resulting in a shift of the gain spectrum toward the shorter wavelengths (i.e., away from the less stable wavelength range). In one embodiment, the reflectivity is reduced by angling the reflector at the end of the resonant cavity of the laser away from a high reflectivity position. Typically, an angle of about one to five degrees relative to a surface perpendicular to a longitudinal axis of the fiber is sufficient. If the output end of the fiber core defines the reflective end of the cavity, the fiber may be cleaved or polished such that the core is at the desired angle. In a different version of this approach, a coating may be applied to the end of the fiber that reduces the reflectivity of the polished or cleaved fiber end to optical energy at the signal wavelength, and thereby reduces signal reflectivity below the glass-to-air Fresnel reflection level.
Another way that the invention may shift the nominal gain spectrum of the laser away from the undesired wavelength range is by increasing the population inversion of the doped core. One approach to doing this is to increase the absorption of pump energy. In one embodiment, a pump energy reflector is provided at the output end of the fiber. The pump reflector reflects pump energy from the inner cladding back through the inner cladding in the opposite direction, thus allowing the absorption of pump energy that was not initially absorbed by the doped core. The pump energy reflector may be a mirror affixed to the end of the fiber that is reflective at the wavelength of the pump energy. Preferably, it does not provide a strong reflection of signal energy from the core. The mirror may therefore be annular, such that it does not intersect the core. A separate low-level reflector for light in the signal wavelength band could be used in this embodiment to provide the desired stabilization of signal energy in the core. In an alternative to the annular mirror, the reflective material may cover the entire end of the fiber, but have a relatively low reflectance at the wavelength of the signal energy. For this embodiment, a coating material may be used that is applied to the end of the fiber.
Other parameters of the apparatus may also be configured to increase the population inversion of the laser. For example, the overall length of the laser fiber may be made short relative to a fiber laser having similar operational parameters. This requires a higher gain and, therefore, a higher population inversion along the fiber to reach laser threshold. The population inversion in the core may also be increased by decreasing the diameter of the inner cladding.
As mentioned above, the shift of the gain spectrum into the undesired wavelength range may also be accomplished by minimizing the shifting of the gain spectrum due to temperature. In an alternative embodiment of the invention, the fiber laser, or at least a portion of it, is maintained in a thermally controlled environment. A temperature-controlled housing may be used that prevents any significant change in the operating temperature of the laser, and therefore prevents any significant shift in the gain spectrum. In the preferred embodiment, the gain spectrum shifts toward the undesired region when the fiber temperature increases. A housing having a cooling mechanism may therefore be used, such as one with an air cooling or water cooling mechanism attached.